The present invention relates to a steering control system for a vehicle with a steered front axle and a steered rear axle, and more particularly, to a steering control system with an anti-lock system acting on all four wheels with a device for intervening in the steering of the rear axle.
A steering control system of the foregoing type is generally already known from Japanese Preliminary Publication 63-215466. A vehicle is equipped with two diagonally acting brake circuits and an anti-lock brake system (ABS) acts on the brake circuits as a reaction to a yawing moment which arises when, during a braking operation, the weight of the vehicle is shifted towards the front wheels. When different brake pressures are built up in the two brake circuits, an intervention in the steered rear axle takes place so that the steering lock counteracts the yawing moment. Thus, if a higher brake pressure is built up in the brake circuit containing the left front wheel than in the other brake circuit, a left-hand yawing moment of the vehicle about the vertical axis of the vehicle arises. In this case, a steering of the rear wheels of the vehicle takes place to the left as a function of the difference between the brake pressures in the two brake circuits, in order to compensate for the yawing moment which has arisen.
A disadvantage of a steering control system of the aforementioned known type is considered to be that stabilization of the driving behavior as a result of an intervention in the rear-axle steering is possible only in the special instance when a brake-pressure difference occurs in a vehicle equipped with diagonally acting brake circuits.
It is known from Japanese Preliminary Publication 62-255282 that the friction coefficient between the wheels of the vehicle and the road surface can be concluded from a comparison between the acceleration acting momentarily on the vehicle and a reference value of the acceleration under reference conditions. The existing relations of the coefficient of friction which are derived from this can be used for controlling a rear-axle steering.
A similar process is known from Japanese Preliminary Publication 62-238171, according to which the output signal of an ABS control unit serves for determining relations of the coefficient of friction between the wheels of the vehicle and the road surface. As a function of these relations of the coefficient of friction, in the case of low values of these relations the rear axle is deflected in-phase with the front wheels or not at all.
In the aforementioned known processes, it can be considered disadvantageous that only the general relations of the coefficient of friction of all the wheels are taken into account in the determination of a suitable steering angle of the rear axle, and that a compensation of a yawing moment arising as a result of .mu.-split condition is not possible.
Furthermore, another steering control system is known from German Auslegeschrift 2,212,328, and in this system the different adhesion coefficients on the two sides of the vehicle are detected by measuring the brake-pressure difference occurring as a result of an anti-lock brake system (ABS). A vehicle yawing moment which has arisen is thus partially compensated for by an intervention in the steered front axle as a function of the brake-pressure difference. This intervention assists the driver's steering movement in order to generate a counter-moment.
A further known method of reducing the yawing moment during braking under .mu.-split conditions is the time-delayed pressure build-up in the brake cylinder of the wheel having the higher adhesion coefficient as disclosed in German Patent Specification 2,518,196. The vehicle yawing moment caused as a result of the braking operating is thereby reduced, so that sufficient time remains for the vehicle driver to keep the vehicle in its track by appropriate counter-steering.
It is known from DE-3,124,821 A1 to design rear-axle steering in such a way that, as a function of the steering lock of the front wheels and of the vehicle speed, the rear wheels are deflected in the same direction as the front wheels, above a specific vehicle speed. Since, when a .mu.-split condition occurs during braking, the vehicle driver steers in the direction of the lower coefficient of friction in order to compensate for the vehicle yawing moment which arises, the set rear-axle steering angle is no longer the best possible during braking under this .mu.-split condition, this resulting in an intensification of the instability in the driving behavior of the vehicle.
An object of the present invention is to provide a steering control system so that the tracking of the vehicle is improved in such a way that the lateral guiding force is influenced and that, as a result, a stabilization of the braking and/or accelerating behavior is obtained. Another object of the present invention is to assure that the intervention of the steering control system takes place at a time when the vehicle has not yet assumed an unstable driving state.
In accordance with the present invention, a steering control system achieves the above objects by a detector for detecting different adhesion coefficients of at least two wheels of different sides of a vehicle, a device for intervening in the steering of the rear axle as a function of a difference between adhesion coefficients, and equipping the vehicle with a drive slip control (ASR) acting at least on the brakes of the driven wheels of the vehicle, regardless of whether there is an ABS signal or an ASR signal so that the intervening device causes a deflection of the rear wheels in the direction of the vehicle sides on which the higher brake torque is generated.
The provision of the steering intervention in the rear axle permits a simpler construction of the separation of the controlled steering intervention from the steering intervention of the vehicle driver in contrast to the known steering control system according to previously discussed German Auslegeschrift 2,212,328, in which the intervention of the vehicle driver and of the control take place at the same axle. Furthermore, the increase in the lateral guiding force of the rear wheels at the initial stage of the critical driving state ensures better stabilization of the tracking of the vehicle rear.
During the braking of a conventional vehicle without rear-axle steering under a .mu.-split condition, a yawing moment arises in the direction of the higher adhesion coefficient and has to be compensated for by a correspondingly pronounced counter-steering. This counter-steering achieves the balance of forces and torques in such a way that the vehicle does not execute any rotational movement and moreover does not experience any lateral shift.
The steering of the rear wheels in the direction of the vehicle side on which the higher braking torque is generated assists the counter-moment generated as a result of the steering movement of the vehicle driver and thereby reduces the total necessary angle of lock of the steering wheel which is required for tracking of the vehicle.
The same result is also obtained in cornering, with the centrifugal force being taken into account in the force and torque balance equation. At the same time, the angle of lock of the rear wheels is to be proportional to the difference of the adhesion coefficients. The time taken for this angle to be obtained is not to exceed approximately 150 ms during rapid braking. In general, this time is not to exceed the duration of the pressure build-up in the main brake cylinder by any more than 150 ms.
The steering movement of the rear wheels leads to an increase in the lateral guiding force, so that the principle of the "select-low" control can be waived in the ABS. As a result, the brake pressure can be built up more quickly in comparison with non-steered rear wheels. The braking travel is thus shortened as a result of this intervention of the steering control system.
The same relationships are obtained when the vehicle is equipped with a drive-slip control (ASR). In the event of a .mu.-split condition, a yawing moment arises during the acceleration of the vehicle. The steering movement of the vehicle driver in order to generate the counter-moment is assisted by a steering angle of the rear wheels in the direction of the vehicle side on which the higher braking torque is generated by the ASR. The size of the steering angle and the dynamics of the steering operation are similar to the relationships which are explained in the description of the braking operation. The steering movement of the rear wheels leads to an increase of the lateral guiding force without a rotation of the vehicle body, so that the tracking of the vehicle is stabilized.
The term "ABS/ASR system" signifies hereafter that an ABS system and/or an ASR system is installed in the vehicle. The different adhesion coefficients during the braking operation are detected by the ABS system, and the different adhesion coefficients during the acceleration operation are detected by the ASR system. If both ABS and ASR systems are installed, a stabilization of the braking and accelerating behavior is achieved, whereas if only one system (ABS or ASR) is installed a corresponding stabilization either of the braking or of the acceleration behavior is achieved.
The detection of the different adhesion coefficients on the two side of the vehicle is carried out by evaluating the control signals of the ABS/ASR system and/or their effects. The control signals of the ABS/ASR system are obtained from the evaluation of the speed sensors of the individual wheels (a cornering of the vehicle has to be taken into account, where appropriate, here). The control signals of the ABS/ASR system lead to different brake pressures in the wheel-brake cylinders, and where the two systems are concerned the following must be remembered:
ABS: Higher brake pressure on the side of the higher adhesion coefficient. PA1 ASR: Higher brake pressure on the side of the lower adhesion coefficient.
During both braking and acceleration, the steering lock of the rear wheels takes place in the direction of the vehicle side on which the higher braking torque is generated. The effects of the control signals from the ABS/ASR system can be:
1. Pressure differences in the wheel-brake cylinders of different sides of the vehicle; these pressure differences can be measures by known differential-pressure sensors or by absolute pressure measurements with subsequent differentiation.
2. Difference between the moments on the wheels; these moments can be determined by torque-measuring hubs or by contactless torque measurements.
3. Difference between the forces on the brake callipers; the braking operating leads to a shift of the brake callipers out of the position of rest and to a deformation relative to the position of rest. The shift or deformation can be detected by known methods of length measurement.